By adopting MIMO (Multiple Input and Multiple Output) technology, an additional spatial degree of freedom may be generated to exponentially increase system capacity. According to theories and practices, with linear increase of the number of transmitting and receiving antennas, MIMO system capacity also increases linearly, thereby greatly enhancing utilization efficiency of a frequency spectrum. Therefore, the MIMO technology has been widely used in existing wireless communication systems, for example, Wi-Fi (Wireless Fidelity), WCDMA (Wideband Code Division Multiple Access), LTE (long Term Evolution) and the like.
However, the conventional MIMO technology requires that each antennas needs one RF (Radio Frequency) link, to transmit different data streams on different antennas, so the cost is relatively large. On the other hand, in order to ensure independent fading characteristics (i.e., a channel matrix is at a good state) of a wireless channel, wavelength between the transmitting antennas should be ensured to be 0.5 wavelength minimally. But for some devices sensitive to sizes (e.g., a miniature terminal), application range of the conventional MIMO technology is limited. To solve problems of cost and size, a technician proposes a new MIMO technology based on an ESPAR (Electronically Steerable Parasitic Array Radiator) antenna, commonly referred to as Single RF MIMO (single radio frequency multiple-input and multiple-output) technology, which includes following features: 1, it consists of an active antenna and a plurality of parasitic antennas, and only one RF link is needed, so that cost is low and structure is simple; 2. one data stream is transmitted on the active antenna, and other data streams are transmitted by a coupled electromagnetic field of the parasitic antennas and the active antenna, thus a plurality of data streams may be transmitted simultaneously; 3, even if a distance between the antennas is smaller than ½ wavelength, it still ensures good independent fading characteristics of a channel, thus being suitable for small space devices. In general, the Single RF MIMO technology effectively avoids the two application shortcomings of the aforementioned MIMO technology.
The ESPAR antenna consists of M+1 units, wherein one unit is the active antenna and is connected with the RF link, and the remaining M units form a parasitic antenna array and are respectively connected with a controllable load. The existing Single RF MIMO technology will be briefly illustrated below with the ESPAR antenna with three units as an example.
As shown in FIG. 1, in the ESPAR antenna with three units, a cylindrical antenna filled with black at the middle is an active antenna and is connected with a RF link, rest two antennas are parasitic antennas and are connected with a parasitic reactor, wherein a value of the parasitic reactance is controlled by a control circuit. A distance between the active antenna and each parasitic antenna is assumed to be d, a data stream s1 is loaded to the active antenna via the RF link for transmitting, while another path of data streams s2 is used for adjusting the reactance values of the parasitic antennas via the control circuit combined with s1, in order to adjust a mutual coupling electromagnetic field between the active antenna and each parasitic antenna, and the data stream s2 is transmitted finally. A transmission directivity diagram G(θ) may be modeled as:
                              G          ⁡                      (            θ            )                          =                ⁢                                            g              isol                        ⁡                          (              θ              )                                *          AF                                        =                ⁢                                            g              isol                        ⁡                          (              θ              )                                *                      a            ⁡                          (              θ              )                                *          i                                        =                ⁢                                            g              isol                        ⁡                          (              θ              )                                *                                                    [                                                                            1                                                                                      ⅇ                                                                              -                            j                                                    ⁢                                                                                                          ⁢                          kd                          ⁢                                                                                                          ⁢                                                      cos                            ⁡                                                          (                                                              θ                                -                                0                                                            )                                                                                                                                                                                          ⅇ                                                                              -                            j                                                    ⁢                                                                                                          ⁢                          kd                          ⁢                                                                                                          ⁢                                                      cos                            ⁡                                                          (                                                              θ                                -                                π                                                            )                                                                                                                                                                          ]                            ⁡                              [                                                                                                    I                        0                                                                                                            I                        1                                                                                                            I                        2                                                                                            ]                                      T                                                  =                ⁢                                            g              isol                        ⁡                          (              θ              )                                *                                                    [                                                                            1                                                                                      ⅇ                                                                              -                            j                                                    ⁢                                                                                                          ⁢                          kd                          ⁢                                                                                                          ⁢                                                      cos                            ⁡                                                          (                              θ                              )                                                                                                                                                                                          ⅇ                                                  j                          ⁢                                                                                                          ⁢                          kd                          ⁢                                                                                                          ⁢                                                      cos                            ⁡                                                          (                              θ                              )                                                                                                                                                                          ]                            ⁡                              [                                                                                                    I                        0                                                                                                            I                        1                                                                                                            I                        2                                                                                            ]                                      T                              
In the above-mentioned formula, gisol(θ) represents the transmission directivity diagram when only a single antenna exists;
AF represents an array factor of the antenna;
k=2π/λ, wherein parameter represents a wavelength;
d represents the interval between the active antenna and the parasitic antenna;
θ represents a departure angle of radiation;
I0, I1, I2 sequentially represent current of three antennas.
Expansion is made via an euler formula to obtain:
                    AF        =                ⁢                              I            0                    +                                    I              1                        ⁢                          ⅇ                                                -                  j                                ⁢                                                                  ⁢                kd                ⁢                                                                  ⁢                                  cos                  ⁡                                      (                    θ                    )                                                                                +                                    I              2                        ⁢                          ⅇ                              j                ⁢                                                                  ⁢                kd                ⁢                                                                  ⁢                                  cos                  ⁡                                      (                    θ                    )                                                                                                              =                ⁢                              I            0                    +                                    (                                                I                  1                                +                                  I                  2                                            )                        ⁢                          cos              ⁡                              (                                  kd                  ⁢                                                                          ⁢                                      cos                    ⁡                                          (                      θ                      )                                                                      )                                              +                                    j              ⁡                              (                                                      I                    2                                    -                                      I                    1                                                  )                                      ⁢                          sin              ⁡                              (                                  kd                  ⁢                                                                          ⁢                                      cos                    ⁡                                          (                      θ                      )                                                                      )                                                                            =                ⁢                              I            0                    (                                                    B                0                            ⁡                              (                θ                )                                      +                                                                                I                    1                                    +                                      I                    2                                                                    I                  0                                            ⁢                                                B                  0                  ′                                ⁡                                  (                  θ                  )                                                      +                          j              ⁢                                                                    I                    2                                    -                                      I                    1                                                                    I                  0                                            ⁢                                                B                  1                                ⁡                                  (                  θ                  )                                                                        
In the above-mentioned formula, B0(θ)=1, B′0(θ)=cos(kd cos(θ)), B1(θ)=sin(kd cos(θ));
when scatterers are sufficient enough, B0(θ)≈cB′0(θ), wherein c≈0.9612; B0(θ)⊥B1(θ). Therefore, an array factor AF may be represented as a linear combination of two paths of orthorhombic basis functions B0(θ) and B1(θ), and the array factor AF may be further simplified as:
                              AF          =                                    I              0                        (                                                            (                                      1                    +                                                                                                                        I                            1                                                    +                                                      I                            2                                                                                                    I                          0                                                                    ⁢                      c                                                        )                                ⁢                                                      B                    0                                    ⁡                                      (                    θ                    )                                                              +                              j                ⁢                                                                            I                      2                                        -                                          I                      1                                                                            I                    0                                                  ⁢                                                      B                    1                                    ⁡                                      (                    θ                    )                                                                                                                    =                                    s              1                        ⁡                          (                                                                    B                    0                                    ⁡                                      (                    θ                    )                                                  +                                                      R                    1                                    ⁢                                                            B                      1                                        ⁡                                          (                      θ                      )                                                                                  )                                                wherein    ,                  ⁢                  s        1            =                        I          0                ⁡                  (                      1            +                                                                                I                    1                                    +                                      I                    2                                                                    I                  0                                            ⁢              c                                )                    represents the first path of transmitted data streams and is modulated by adjusting I0; and
the coefficient 1 is adjusted:
            R      1        =          j      ⁢                                                  I              2                                      I              0                                -                                    I              1                                      I              0                                                1          +                                    (                                                                    I                    1                                                        I                    0                                                  +                                                      I                    2                                                        I                    0                                                              )                        ⁢            c                                ,          ⁢      values    ⁢                  ⁢    of    ⁢                  ⁢                  I        2                    I        0              ⁢                  ⁢    and    ⁢                  ⁢                  I        1                    I        0            are controlled by controlling jX1 and jX2 so that
            R      1        =          r      ⁢                        s          2                          s          1                      ;the parameter r is used for balancing the power of the basis functions, and when r=3.67, P(B0(θ))=r2P(B1(θ)). Thus, AF=s1B0(θ)+rs2B1(θ).
On the other hand,
            I      2              I      0        ⁢          ⁢  and  ⁢          ⁢            I      1              I      0      may be calculated by the following method:V0=I0Z00+I1Z01+I2Z02 −jI1X1=I0Z10+I1Z11+I2Z12 −jI1X2=I0Z20+I1Z21+I2Z22  (1)
wherein, Zii, i=0, 1, 2 represents self-impedance of the three antennas; Zij, i≠j represents mutual impedance of the antenna i and the antenna j; the following formulas can be obtained from formula (1):
                                          I            1                                I            0                          =                                                            Z                12                            ⁢                              Z                02                                      -                                          Z                01                            ⁡                              (                                                      Z                    22                                    +                                      j                    ⁢                                                                                  ⁢                                          X                      2                                                                      )                                                                                        (                                                      Z                    11                                    +                                      jX                    1                                                  )                            ⁢                              (                                                      Z                    22                                    +                                      jX                    2                                                  )                                      -                          Z              12              2                                                          (        2        )                                                      I            2                                I            0                          =                                                            Z                12                            ⁢                              Z                02                                      -                                          Z                02                            ⁡                              (                                                      Z                    11                                    +                                      j                    ⁢                                                                                  ⁢                                          X                      1                                                                      )                                                                                        (                                                      Z                    11                                    +                                      jX                    1                                                  )                            ⁢                              (                                                      Z                    22                                    +                                      jX                    2                                                  )                                      -                          Z              12              2                                                          (        3        )            
it can be seen from formula (2) and formula (3) that, s2/s1 is obtained according to input of s1 and s2, jX1 and jX2 are adjusted to change
                    I        2                    I        0              ⁢                  ⁢    and    ⁢                  ⁢                  I        1                    I        0              ,to obtain
            R      1        =          r      ⁢                        s          2                          s          1                      ,and accordingly, two data streams are transmitted simultaneously. BPSK (Binary Phase Shift Keying) modulation
Since s1 and s2 are selected in the set {−1,1}, the ratio, s2/s1 is selected in the set {-1,1}. It is assumed that, jX1 and jX2 respectively change from −100j to −0.4j, and minimum step size is −0.2j; when r=3.67, if amplitude deviation satisfies
            E      Ampli        =                                                              R              1                        -            r                    r                            ≤      0.04        ,and angle deviation satisfies
            E      Angle        =                                                ∠            ⁢                                                  ⁢                          R              1                                -                      ∠            ⁢                                                  ⁢                                          s                2                                            s                1                                                                ≤      0.02        ,exhaustive search is stopped to output a corresponding parasitic reactance combined value (note: the exhaustive search is performed only once, as it is irrespective to channel achievement), as shown in the following table 1:
TABLE 1Parasitic Reactance Combined Value of BPSKs1/s0X1X21−3.8−8.0−1−8.0−3.8
QPSK (Quadrature Phase Shift Keying) modulation:
Since s1 and s2 are selected in the set (1+j,−1+j,−1−j,−1+j), the ratio, s2/s1, is selected in the set {1,−1,j,−j}. It is assumed that, jX1 and jX2 respectively change from −100j to −0.4j, and minimum step size is −0.2j; when r=3.67, if amplitude deviation satisfies
            E      Ampli        =                                                              R              1                        -            r                    r                            ≤      0.04        ,and angle deviation satisfies
            E      Angle        =                                                ∠            ⁢                                                  ⁢                          R              1                                -                      ∠            ⁢                                                  ⁢                                          s                2                                            s                1                                                                ≤      0.02        ,exhaustive search is stopped to output a corresponding parasitic reactance combined value, as shown in the following table 2:
TABLE 2Parasitic Reactance Combined Value of QPSKs1/s0X1X2  1−3.8−8.0−1−8.0−3.8−j−33.8−51.8  j−51.8−33.6
16QAM Modulation (quadrature amplitude modulation of 16 symbols):
Please see FIG. 2(a), FIG. 2(b) and FIG. 3, constellation points of s1 or s2 are selected in FIG. 2(a), and correspondingly, 52 constellation points of s2/s1 may be as shown in FIG. 2(b); it is assumed that, jX1 and jX2 respectively change from |100j to −0.4j, minimum step size is 0.2j; when r=3.67, if amplitude deviation satisfies
            E      Ampli        =                                                              R              1                        -            r                    r                            ≤      0.04        ,and angle deviation satisfies
            E      Angle        =                                                ∠            ⁢                                                  ⁢                          R              1                                -                      ∠            ⁢                                                  ⁢                                          s                2                                            s                1                                                                ≤      0.02        ,exhaustive search is stopped to obtain the constellation points of R1/3.67 as shown in FIG. 3. It can be seen from the figure that, there are 8 points for which corresponding parasitic reactance combinations for satisfying the amplitude deviation and the angle deviation respectively can not be found. Even if r=2.5, at this time, power allocation of the basis functions is severely unequal, P(B0(θ))>r2P(B1(θ)), and at this time, it may be still found from simulation that the corresponding parasitic reactance combinations of 4 constellation points could not be found. In FIG. 2(b), transverse and longitudinal distributions are: in-phase component (In-Phase), quadrature component (Quadrature). In FIG. 2(a), FIG. 2(b), FIG. 3, FIG. 5 and FIG. 6, triangles are constellation points of R1/3.67, and asterisks are constellation points of s2/s1, which will not be illustrated repeatedly in the following embodiments. It can be seen from the above analysis that, in an open loop Single RF MIMO transmission process, when the same power is allocated for the basis functions, the system capacity is optimal. Therefore, in high order modulation (for example, 16QAM modulation) of Single RF MIMO, in order to find out the parasitic reactance combinations corresponding to 52 constellation points, the power allocated to the first basis function is much larger than the power of the second basis function, thereby greatly reducing the system capacity, and similar analysis also exists in 16PSK modulation. In the prior art, the high order modulation of Single RF MIMO could not be supported. Three solutions are proposed in the literature, but various problems are still present: 1. adjustable negative resistance is introduced, but this will break the stability of the system; 2. adjustable positive resistance is introduced, but this will dissipate a part of power of the system; 3. pre-coding is introduced, but this will worsen the computational burden of a transmitter or a receiver.